Improvements of charged particle beam devices, like electron microscopes or focused ion beam devices (FIB), often depend on improvements of their beam optical components. Beam optical components are, for example, electrostatic or magnetic lenses, deflectors, electrostatic or magnetic mirrors, spectrometers and the like.
Beam optical components that rely on an electrostatic interaction with the charged particle beam are usually composed of two or more electrodes with openings or some other structure thereon. By applying appropriate voltages to the respective electrodes, the geometric shape of the electrodes and the electric potentials generate an electrostatic field that can be used to focus, deflect or disperse an incoming charged particle beam.
Contrastingly, beam optical components that rely on a magnetic interaction with the charged particle beam are composed of two or more pole pieces with openings or some other structure thereon. By applying an appropriate magnetic flux to the respective pole pieces, the geometric shape of the pole pieces and the magnetic flux generate a magnetic field that, similar to an electric field, can be used to focus, deflect or disperse an incoming charged particle beam.
In order to provide a well-defined focussing, deflection or dispersion, it is important that the multiple electrodes or pole pieces are well aligned with respect to each other. For example, for an electrostatic or magnetic lens made of two electrodes or two pole shoes, it is important that the openings of the first electrode or first pole shoes are coaxially aligned with respect to the openings of the second electrode or pole shoes with a precision on a micrometer scale. To achieve this precision usually represents a major challenge.
Further, beam optical components in high-precision charged particle beam devices have to be cleaned on a regular basis in order to perform to their specifications. An effective cleaning, however, requires that the beam optical component has to be disassembled and re-assembled again. Therefore, an beam optical component must comply with a design that allows for a repetitive disassembly and re-assembly without harming the alignment precision.
A method for manufacturing an electrostatic lens with high alignment precision is disclosed by S. Planck and R. Spehr in “Construction and fabrication of electrostatic field lenses for the SMART project” in the Annual Report 1996/1997 of “Licht- und Teilchenoptik”, Institut für angewandte Physik, Technische Unversität Darmstadt, Prof. Dr. Theo Tschudi on page 114. In this report, it is disclosed that six electrically insulating Al2O3 spheres between the middle electrode of an Einzel-lens and the two outer electrodes are used as positioning elements to define the positions of the two outer electrodes with respect to the middle electrode. At the same time, the insulating character of the Al2O3 spheres provides that the middle electrode is electrically insulated from the two outer electrodes in order to be able to apply different voltages to the electrodes.
FIG. 1 schematically illustrates a cross section through the known electrostatic lens 1 by S. Planck, used for high-precision charged particle beam optics. The electrostatic lens 1 comprises a first electrode 3, a second electrode 5, and a third electrode 7, each electrode having respective first, second and third openings 9, 10, 11. First and second electrodes 3, 5 are kept at a first predetermined minimum distance by three equal first spheres 120 which are positioned between the first electrode 3 and the second electrode 4, while second and third electrodes 5, 7 are kept at a second predetermined minimum distance by three equal second spheres 122 between the second electrode 5 and the third electrode 7. Further, three metal screws 100 are used to clamp the three electrodes 3, 5, 7 together.
The high precision of the alignment of the three openings 9, 10, 11 of the electrodes with respect to the optical axis 13 is based on the high geometrical precision by which spheres can be manufactured. For example, it is known to manufacture spheres made of steel or Al2O3 with a precision that deviates by less than a micrometer from a specified ideal spherical shape. Further, recesses in the electrodes for receiving the spheres provide for an easy and precise repositioning of the spheres during re-assembly of the beam optical component.
However, when tightening the screws during re-assemblage of the beam optical component to clamp the electrodes together, the electrodes often become distorted or tilted with respect to each other, which diminishes the focussing quality of the beam optical component.
Further, as can be seen from FIG. 1, it is difficult to prevent arcing between second electrode 5 and metal screw 100 when high voltages are applied between second electrode 5 and first or third electrode 3, 5, due to the limited size of through-hole 102 in the second electrode. Further, for many applications, it is important to apply different voltages between the two outer (first and third) electrodes. Since screws made of an insulating material usually do not have a stiffness that a metal screw has, metal screws are usually taken for clamping the three electrodes together. With a metal screw clamping the three electrodes together, however, first and third electrode 3, 7 would electrically shorten first and second electrodes when different voltages would be applied.
The electrostatic lens 1 of FIG. 1 is only used as an example for demonstrating the general alignment problems of electrostatic or magnetic beam optical components in charged particle beam devices. Similar alignment problems of two or more electrodes or pole shoes also occur when two or more electrodes or pole shoes of an electrostatic or magnetic mirror or a spectrometer have to be assembled.